Fire protection systems of the sprinkler type are, in principle, of two somewhat different kinds. The first one is the so-called dry pipe system, in which the water is released by a detector-controlled main valve and flows out into the pipes to the various sprinkler heads. Systems of this type are not particularly rapid, since the water has to travel a rather long way before it reaches the sprinkler heads. The other system is the so-called wet pipe system, in which there is already water at the sprinkler heads which, in turn, are provided with separate sealing systems of one kind or another. The delay will then be limited to the action time of the sealing system itself or the system for initiating it. Temperature-sensitive systems based upon this principle have been in rather common use for more than 20 years. These temperature-sensitive sealing or blocking systems are as a rule based upon either low-temperature-sensitive soldering joints or hollow glass bulbs filled with some appropriate chemical substance, primarily a liquid, which at an increased temperature breaks the bulb from the inside and thereby releases the blocking of the outlet nozzle of the sprinkler head.
Both of these blocking systems are considered to be reliable, but altogether too slow for use in premises classified as high piled storage risks, such as in the explosives industry, in cellular plastics and paint factories, and so forth. In such cases, considerably more rapid systems are required, which need not wait for a rise in temperature in the premises.
Today, flame detectors are known which operate within the ultra-violet range, entirely undisturbed by normal sunlight. Such flame detectors do indeed give signals of varying strength for different types of flames, but the signal can easily be amplified so that it can be used for releasing a valve of one kind or another.
In the endeavours to achieve a particularly rapidly released electrically controlled sprinkler valve, it has also been proposed to have the valve released by an electrically initiated detonator. The basic design of the type hitherto tested corresponds to that of the previously mentioned sprinkler head, with the glass bulb complemented with a displaceable piston directed in a guide towards the central part of the bulb which, by the initiation of an electric detonator applied behind the piston, is thrown forwards against the bulb, to crush it. However, experience has shown that this piston must be very accurately controlled in its movement towards the bulb, in order to crush it completely. If the body of the hollow bulb should be crushed on one side only, the remaining parts can continue to block the outlet for the water. It should be noted that the glass bulb must be comparatively strong in order to be able to hold the water pressure, while the dimensions of the piston must be limited and the detonator must have the smallest possible charge.
The piston and its guide must also be made with close tolerances, as the so-called bureau drawer effect is otherwise quite liable to jeopardize the function of the device.